On January 9, 1861, Mississippi became the second state to declare its secession from the Union, and it was one of the founding members of the Confederate States of America.
Their reason was slavery. Following is their declaration:

AN
ORDINANCE to dissolve the union between the State of Mississippi and other
States united with her under the compact entitled "The Constitution of the
United States of America."

The
people of the State of Mississippi, in convention assembled, do ordain and
declare, and it is hereby ordained and declared, as follows, to wit:

In the momentous step which our State has taken of dissolving its
connection with the government of which we so long formed a part, it is but just that we should declare the prominent reasons which have induced our course.

Our position is thoroughly identified with the institution of slavery-- the greatest material interest of the world. Its labor supplies the product which constitutes by far the largest and most important portions of commerce of the earth. These products are peculiar to the climate verging on the tropical regions, and by an imperious law of nature, none but the black race can bear exposure to the tropical sun. These products have become necessities of the world, and a blow at slavery is a blow at commerce and
civilization. That blow has been long aimed at the institution, and was at the point of reaching its consummation. There was no choice left us but submission to the mandates of abolition, or a dissolution of the Union, whose principles had been subverted to work out our ruin.

That we do not overstate the dangers to our institution, a reference to a few facts will sufficiently prove.

It has grown until it denies the right of property in slaves, and refuses protection to that right on the high seas, in the
Territories, and wherever the government of the United States had jurisdiction.

It refuses the admission of new slave States into the Union, and seeks to extinguish it by confining it within its present limits, denying the power of expansion.

It tramples the original equality of the South under foot.

It has nullified the Fugitive Slave Law in almost every free State in the Union, and has utterly broken the compact which our fathers pledged their faith to maintain.

It advocates negro equality, socially and politically, and promotes insurrection and incendiarism in our midst.

It has enlisted its press, its pulpit and its schools against us, until the whole popular mind of the North is excited and inflamed with prejudice.

It has made combinations and formed associations to carry out its schemes of emancipation in the States and wherever else slavery exists.

It seeks not to elevate or to support the slave, but to destroy his present condition without providing a better.

It has invaded a State, and invested with the honors of martyrdom the wretch whose purpose was to apply flames to our dwellings, and the weapons of destruction to our lives.

It has broken every compact into which it has entered for our security.

It has given indubitable evidence of its design to ruin our agriculture, to prostrate our industrial pursuits and to destroy our social system.

It knows no relenting or hesitation in its purposes; it stops not in its march of aggression, and leaves us no room to hope for cessation or for pause.

It has recently obtained control of the Government, by the prosecution of its unhallowed schemes, and destroyed the last expectation of living together in friendship and brotherhood.

Utter subjugation awaits us in the Union, if we should consent longer to remain in it. It is not a matter of choice, but of necessity. We must either submit to degradation, and to the loss of property worth four billions of money, or we must secede from the Union framed by our fathers, to secure this as well as every other species of
property. For far less cause than this, our fathers separated from the Crown of England.

Our decision is made. We follow their footsteps. We embrace the alternative of separation; and for the reasons here stated, we resolve to maintain our rights with the full consciousness of the justice of our course, and the undoubting belief of our ability to maintain it.